1. Technical Field
The present invention relates to a semiconductor device having a trench structure.
2. Background Art
Along with the times, full use of fine processing technology has enabled manufacturing of smaller semiconductor devices without lowering performance. This trend also applies to a semiconductor element having high driving performance. The reduction of an ON resistance of the element per unit area has been achieved with the best use of fine processing technology. However, it is also a fact that lowering of withstanding voltage, which is caused by miniaturization of elements, hinders further improvement of the driving performance. Elements with various structures have been proposed in order to eliminate the trade-off between miniaturization and withstanding voltage. A trench gate metal oxide semiconductor (MOS) transistor is an example in a power MOS field effect transistor (FET) with a high withstanding voltage and a high driving performance, which is a current mainstream. The trench gate MOS transistor has the highest packing density in integration among double diffused MOS (DMOS) transistors having a high withstanding voltage and a high driving performance. The trench gate MOS transistor has, however, a longitudinal MOS structure in which current flows in a depth direction of a substrate. The transistor has extremely excellent performance as an element itself, but has a disadvantage when mounted on a chip with other ICs. When mounting on a chip with other ICs is taken into consideration, a conventional lateral MOS structure still has an advantage. A lateral trench gate transistor has been proposed as a method for further reducing ON resistance per unit area without lowering withstanding voltage. In this transistor, a gate portion has a trench structure having a convex portion and a concave portion to gain a larger gate width (for example, refer to Patent Document 1).
FIGS. 3 and 4 are conceptual views each illustrating a conventional lateral trench gate transistor, in which FIG. 3(a) is a bird's-eye view, FIG. 3(b) is a plan view, and FIGS. 4(a), 4(b), and 4(c) are cross-sectional views taken along the lines 4A-4A′, 4B-4B′, and 4C-4C′ of FIG. 3(b), respectively. The lateral trench gate transistor includes: a plurality of trenches which are formed in parallel to a channel direction of a transistor, the transistor being formed in a p-type well region 005, the p-type well region 005 being formed in a surface of an n-type or p-type high resistance semiconductor substrate 006; concave portions 008 and convex portions 007 which are defined by the trenches, the convex portions 007 being located on both sides of the concave portions; a gate insulating film 004 which is formed on surfaces of the concave portions and the convex portions; a gate electrode 003 which fills the concave portions and is formed on the gate insulating film on the convex portions; and a source region 001 and a drain region 002 which are formed on a surface of the well region on both sides of the gate electrode so as to be interlocked with the trenches. It should be noted in FIG. 3(b) that, for simplicity of the figure, the gate electrode 003 and the gate insulating film 004 are transparent outside of the trenches, and edges of the gate electrode 003 are indicated by thick lines. According to the invention of Patent Document 1, a gate portion has the trench structure, to thereby increase a gate width of a lateral MOS per unit plane area and reduce an ON resistance. Broken lines 019 of FIGS. 3(a) and 4(c) indicate a path of a current flowing through the transistor.
Patent Document 1: JP 2006-294645 A
However, the invention of Patent Document 1 has one problem. In the invention of Patent Document 1, a channel length near a bottom part of the entire trench becomes longer than a channel length near an upper part of the entire trench as the trench becomes deeper and a length LP of an upper part of the gate electrode illustrated in FIGS. 3 and 4 becomes shorter. As a result, there arises a problem that not enough current flows along the current path 019 near the bottom part of the trench as illustrated in FIGS. 3 and 4 and thus a sufficient driving performance may not be obtained.